This invention relates to pneumatically actuated syringes, and more particularly to the plungers used in such syringes.
Pneumatically actuated syringes are used in dispensing controlled amounts of encapsulants, adhesives, including epoxies or other materials onto circuit boards or integrated circuits during the manufacture thereof, as shown and described in U.S. Pat. No. 5,927,560. The liquid filled syringe supplies, or feeds, liquid to a dispensing pump.
In a conventional set up, the dispensing material comes in standard size syringes. The syringe operatively connects to a valve and pump arrangement to dispense the material onto a substrate in precise amounts. The liquid is forced from the syringe to the pump via a plunger. More specifically, fluid pressure applied to the backside of a piston, or plunger, residing within the syringe forces the plunger toward a liquid outlet located on the frontside of the plunger. Movement of the plunger toward the outlet forces liquid out of the outlet, to the dispensing pump.
In the manufacture of circuit boards or integrated circuits, the syringe plunger must drive the liquid material from the syringe outlet in a consistent and precise manner, to help assure that the dispensed amounts, typically called xe2x80x9cshots,xe2x80x9d will be uniform in size and weight throughout the dispensing procedure. That is, to some extent the precision of the syringe plunger is necessary to achieve a high degree of repeatability with respect to the ultimate dispensing of shots of uniform size and weight.
These liquid dispensing materials are quite expensive, typically costing more than $150.00 per ounce. To assure minimum waste of these expensive liquid dispensing materials, and to avoid running out of dispensing liquid during a dispensing procedure, it would be helpful to know with certainty the fluid level in the syringe. One way this could be done is by monitoring the position of the plunger within the syringe. However, this is typically not done in practice.
One reason this is not usually done is because in some instances the plunger position gives an inaccurate indication of fluid level. This may occur as a result of air becoming entrapped between the plunger and the liquid in the syringe. In this condition the plunger is said to xe2x80x9cfloatxe2x80x9d above the liquid. In this floating condition, the position of the plunger does not provide an accurate indication of the amount of liquid left in the syringe. Thus, an operator must guess as to how much liquid remains in the syringe in order to avoid running out. Moreover, with this uncertainty, the operator cannot effectively run the apparatus until the liquid level is known to be empty. As a result, it is inevitable that at least some syringes which still have a usable amount of liquid will be discarded prematurely, representing a waste of this relatively expensive liquid.
Another adverse effect of entrapped air within the syringe is its negative impact on dispensing quality. Air pressure cycling causes volume fluctuations in any air that is entrapped in the syringe. Unlike dispensing fluids, which are incompressible liquids, air contracts under high pressure and expands when pressure is reduced. In some cases, particularly where a continuous fluid path exists between the syringe and the dispensing pump outlet, these volume fluctuations may result in inadvertent drooling, or dripping, of liquid from the dispensing pump outlet. This inadvertent drooling, or dripping, results in wasted liquid and/or liquid being deposited onto undesired areas of the substrate. Furthermore, these volume fluctuations may adversely effect repeatability with respect to dispensing shots of uniform size and weight.
One way air becomes entrapped within the syringe, thereby to xe2x80x9cfloatxe2x80x9d the plunger, results from the outward bulging of the syringe walls when air pressure is applied. For instance, in a pressurized condition, such as 80 psig for the backside air pressure, the walls of the syringe tend to bulge outwardly. Because of this bulging, a circumferential air gap forms between the plunger and the inside surface of the syringe. Typically, these components are made of hard plastic, such as polyethylene, and so there is not an airtight seal therebetween.
Thereafter, when the backside air pressure is reduced to zero psig, the syringe walls contract inwardly. Because there is no circumferential airtight seal at the front end of the plunger, where the plunger contacts the liquid, at least some of the air that has been introduced from this cylindrical air gap moves in front of the plunger, between the plunger and the dispensing fluid. Thus, the syringe wall contraction entraps the air between the plunger and the dispensing liquid, thereby causing the plunger to rise, or float. In some instances where air pressure is frequently cycled, the floating distance may accumulate to be as much as xc2xcxe2x80x3 or even xc2xdxe2x80x3, for a syringe of 1.6xe2x80x3 diameter.
It is therefore an object of the present invention to eliminate the occurrence of a xe2x80x9cfloatingxe2x80x9d plunger condition in pneumatically actuated syringes.
It is another object of the present invention to reduce the possibility of air entrapment between the plunger and the liquid in a pneumatically actuated syringe, thereby to promote repeatability in liquid dispensing and to avoid drool.
It is still another object of the present invention to reduce the amount of liquid which is wasted during the use of liquid filled syringes, by enhancing the accuracy of liquid level sensing within the syringe.
The present invention achieves the above-stated objects via an anti-float syringe plunger having an outer flexible wiper which maintains an airtight seal with the inside surface of the syringe and a forward deformable membrane which deforms to alleviate residual pressure in the syringe when the walls return from their bulged condition.
The flexible wiper maintains a circumferential airtight seal with the inside surface of the syringe. So even when the syringe walls bulge under pressure, the wiper prevents formation of an air gap between the plunger and the inside walls of the syringe. Since an air gap does not form, air should not become entrapped between the piston and the liquid when the syringe walls return from the bulged condition. Thus, the invention eliminates the occurrence of entrapped air which would otherwise cause the plunger to float. Stated another way, the invention eliminates the floating plunger condition.
One result of this airtight seal is an increase in the residual pressure which results from the contraction of the syringe walls when air pressure is released at the backside, In conventional syringe plungers there is no excessive residual pressure buildup due to syringe wall contraction because there is such a loose seal. In contrast, with the plunger/syringe of this invention the residual pressure buildup could otherwise cause liquid drool, or dripping, at the syringe outlet, particularly when used for direct dispensing. However, because of the deformable membrane at the forward end of the plunger, contraction of the syringe walls causes neither inadvertent drooling nor undesired upward movement of the plunger when the syringe walls contract. Rather, the residual pressure which results from contraction of the syringe walls will be alleviated, or absorbed, by deformation of the membrane.
Also, because the plunger of this invention does not float, due to the flexible seal and the deformable membrane, the sensing of plunger position gives an accurate indication of the liquid level. This leads to reduced waste in dispensing liquid.
According to a preferred embodiment of the invention, the plunger includes a rigid cylindrically shaped ring, preferably of plastic, with an outer cover, preferably of silicon rubber, extending thereover. The inner ring may have an inner shelf to support a magnet for sensing purposes. The cover is preferably made in a unitary manner by molding an elastomeric material into a cylindrical shape which is closed at one end. The inner ring is inserted within the cover. The cover includes a radially outwardly extending flexible wiper traversing its entire circumference. Preferably, the cover includes two spaced flexible wipers, a first located at the forward end and a second located at the rear end of the plunger. The second wiper, however, does not extend completely around the cover and does not form a circumferential airtight seal with the syringe walls. Primarily, the second wiper helps to maintain the plunger centered within the syringe.
The closed end of the cover includes the deformable membrane, which is arcuately shaped in cross section. The cover remains in direct contact with the liquid in the syringe regardless of the backside pressure, due to its flexibility and its shape. The present invention may be used advantageously in dispensing various types of liquids, including but not limited to adhesives, sealants, encapsulants, coatings, epoxy, thermal grease, etc.
These and other features of the invention will be more readily understood in view of the following detailed description and the drawings.